# DC-DC Converter

The power losses in a power electronic DC-DC converter can be divided into conduction and switching losses, where conduction losses consist of inductor conduction losses and MOSFET conduction losses [28] (Fig.14.7).

• Inductor Conduction Losses Inductor conduction losses is as follows

Pl = I2L x Rl {14.17)

where Rl is the DC-Resistance of the inductor,

The inductor rms current (IL):

D/2

11 = /2 + (14.18)

where /o the output current and D/ the ripple current.

Typically, D/ is about 30 % of the output current. Therefore, the inductor current can be calculated as:

/l = /o x 1.00375 (14.19)

Because the ripple current contributes only 0.375 % of IL, it can be neglected. The power dissipated in the inductor now can be calculated as:

Pl = /o2 x Rl

• Power Dissipated in the MOSFETs

The power dissipated in the high-side MOSFET is given by:

PQ1 = /2nnS_Q1 x RDSON1 (14-2ї)

where Rdson1 is the on-time drain-to-source resistance of the high-side MOSFET.

where Rdson2 is the on-time drain-to-source resistance of the low-side MOSFET.

and:

L = Inductance (H) f = Frequency (Hz)

Table 14.5 Investigated appliances, appliance ratings and estimated on time per day

 Product name Quantity Ratings Power rating (W) Current rating (A) Voltage Rating (V) Appliance on time in a day Light bulb LED 4 7 0.60 12 DC 10 Light bulb CFL 2 12 1.00 12 DC 10 Microwave oven 1 235 10.00 24 DC 1 Induction stove 1 2000 10.00 230 AC 2 Electric geyser 1 1500 8.00 230 AC 1 Sandwich maker 1 550 23.00 24 DC 0.5 Coffee maker 1 135 11.00 12 DC 0.5 Refrigerator 1 72 3.00 24 DC 12 (DC) Water purifier 1 11 0.50 24 DC 1 Ventilation fan 4 20 0.90 24 DC 5 Submersible 1 240 10.00 24 DC 0.5 pump Washing 1 70 3.00 24 DC 0.5 machine Vacuum cleaner 1 95 8.00 12 DC 0.25 Window unit AC 2 800 33.30 24 DC 12 Laptop 1 65 3.34 19.5 DC 7 Perdonal 1 170 14.00 12 DC 5 computer External modem 1 5 0.43 12 DC 24 15.6’’LCD 1 30 2.50 12 DC 5 television Ceiling fan 4 20 1.70 12 DC 4 Hair dryer 1 425 15.00 24 DC 0.5 Cell phone 4 4 0.30 12 DC 5 Hybrid car 1 3000 12.50 240 DC 10

VIN = Input voltage (V) Vo = Output voltage (V)

• MOSFET Conduction Losses

 Product Name Cable Size (mm2) Cable length (meter) Energy Loss in feeder (kWh/yr) Total Energy Consumption (kWh/yr) Light Bulb LED 4 40 0.23 102.43 Light Bulb CFL 4 30 0.47 88.07 Microwave oven 6 10 1.05 86.82 Induction Stove 1.5 20 16.74 1476.74 Electric Geyser 1.5 30 8.04 555.54 Sandwich Maker 35 20 0.95 101.32 Coffee Maker 25 20 0.30 24.94 Refrigerator DC 4 20 3.39 318.75 Water Purifier 1.5 20 0.02 4.04 Ventilation fan 1.5 30 0.51 146.51 Submersible Pump 6 10 0.52 44.32 Washing Machine 6 25 0.12 12.89 Vacuum Cleaner 16 15 0.09 8.76 Window Unit AC 25 10 33.42 7041.42 Laptop 10 40 1.96 168.04 Personal Computer 25 15 3.69 313.94 External Modem 1.5 30 0.56 44.36 15.6’’ LCD Television 6 20 0.65 55.40 Ceiling Fan 4 20 0.36 117.16 Hair Dryan 16 15 0.66 78.22 Cell Phone 1.5 20 0.04 29.24 Hybrid Car 1.5 10 65.40 11015.40 Total Energy in kWh/year 139.17 21834.31
 Table 14.6 Cable size, power loss and energy consumption for DC system

For typical buck power supply designs, the inductor’s ripple current, DI, is less than 30 % of the total output current, so the contribution of DI2/12 to the is negligible and can be dropped to get:

Note that when RsDON1 = RsDON2, then:

PFET = iO x rsdon2 (14.29)

The power dissipated in the MOSFET is independent of the output voltage. By using Eq. (14.29) the conduction losses of MOSFET can be calculated at any output voltage. On the other side, inductor conduction losses and switching losses etc. are independent of output voltage and remains constant with change in output voltage [22].

Hence, PD now can be computed as:

PD = PL + Pfet + otherlosses (14.30)

There are some other types of losses such as the MOSFET switching losses, quiescent current etc. At any output voltage, the overall efficiency can be calcu­lated by the known total power supply losses and power supply output power.

Updated: October 23, 2015 — 12:41 pm